Method of producing a glass substrate for a magnetic recording medium and method of producing a magnetic recording medium

ABSTRACT

In a method of producing a glass substrate for a magnetic recording medium, including the steps of grinding a principal surface of the glass substrate of a disk-shape and polishing, after the grinding step, the principal surface of the glass substrate by an abrasive liquid including abrasive grains, the abrasive liquid includes colloidal silica abrasive grains as the abrasive grains and further includes an alkali so that a pH value of the abrasive liquid is adjusted to be greater than 10.2 and not greater than 12.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method of producing a glass substrate for a magnetic recording medium adapted to high-density recording/reproducing and a method of producing a magnetic recording medium.

[0002] In recent years, there is a standing demand for a magnetic recording medium having a higher recording density. In order to achieve the higher recording density of the magnetic recording medium, it is important to reduce a flying height of a magnetic head with respect to a surface of the magnetic recording medium. The flying height of the magnetic head has a correlation with the surface roughness of the magnetic recording medium. Therefore, it is attempted to smooth the surface roughness of the magnetic recording medium and to smooth the surface roughness of a substrate for the magnetic recording medium.

[0003] As the substrate for the magnetic recording medium, use has been made of a glass substrate because of high mechanical durability and high smoothness. For example, methods of smoothing the glass substrate are disclosed in Japanese Unexamined Patent Publications JP 7-240025 A and JP 10-241144 A.

[0004] JP 7-240025 A discloses a method of producing a substrate for a magnetic disk. In this method, a removing step (polishing step) is carried out by the use of a colloid particle (colloidal silica) aqueous solution having a pH value adjusted to exhibit an acidity.

[0005] JP 10-241144 A discloses a method of producing a glass substrate for a magnetic recording medium. In this method, a grinding step is followed by first and second polishing steps using an abrasive liquid containing cerium oxide and water and a third polishing step using an abrasive liquid containing colloidal silica and water.

[0006] However, the former method uses the acidic aqueous solution as an abrasive liquid and therefore suffers slurry flocculation and corrosion (oxidization) of a polishing plate. The latter method includes the three-stage polishing steps in order to achieve high smoothness of the surface of the glass substrate and therefore requires high production cost. In the third polishing step, the abrasive liquid of colloidal silica and water is used. Therefore, the polishing rate is slow so that the polishing time is long and the productivity is degraded. In a cleaning step after the polishing steps, ultrasonic cleaning is performed using water or an alkali aqueous solution. In case of cleaning with water, colloidal silica abrasive grains can not sufficiently be removed so that an abrasive residue is produced. In case of cleaning with the alkali aqueous solution (which typically has a concentration of 7 wt % (pH of 14.243)), the colloidal silica abrasive grains are dissolved and removed so that no abrasive residue is produced. However, if the concentration of the alkali aqueous solution is increased or the cleaning condition (application of the ultrasonic wave) is enhanced in order to remove the colloidal silica abrasive grains, the glass substrate suffers a damage (concave defect) due to the alkali aqueous solution. The concave defect will cause a signal error if the magnetic recording medium is produced by forming at least a magnetic layer on the substrate and recording/reproducing operations are carried out upon the magnetic recording medium.

SUMMARY OF THE INVENTION

[0007] It is an object of this invention to provide a method which is capable of producing a glass substrate for a magnetic recording medium having a high smoothness without an abrasive residue and an concave defect and which is excellent in productivity.

[0008] It is another object of this invention to provide a method of producing a magnetic recording medium which is capable of avoiding a signal error due to presence of an concave defect on a glass substrate and which is excellent in productivity.

[0009] Other objects will become clear as the description proceeds.

[0010] In order to achieve the above-mentioned objects, this invention has following structures.

[0011] Structure 1

[0012] A method of producing a glass substrate for a magnetic recording medium, comprising the steps of grinding a principal surface of the glass substrate of a disk-shape and polishing, after the grinding step, the principal surface of the glass substrate by an abrasive liquid including abrasive grains, wherein the abrasive grains are colloidal silica abrasive grains, the abrasive liquid comprising an alkali so that a pH value of the abrasive liquid is adjusted to be greater than 10.2 and not greater than 12.

[0013] Structure 2

[0014] A method according to structure 1, further comprising the step of carrying out, after the polishing step, alkali cleaning of the principal surface of the glass substrate.

[0015] Structure 3

[0016] A method according to structure 2, wherein the same alkali component is used as the alkali included in the abrasive liquid and an alkali used in the alkali cleaning.

[0017] Structure 4

[0018] A method according to structure 1, wherein the glass substrate is made of an aluminosilicate glass.

[0019] Structure 5

[0020] A method of producing a magnetic recording medium, comprising a step of forming at least a magnetic layer on the principal surface of the glass substrate which is obtained by the method according to structure 1.

[0021] According to the above-mentioned structure 1, a fine-particle abrasive, i.e., the colloidal silica abrasive grains is used as the abrasive liquid so that a high smoothness is achieved. Furthermore, the abrasive liquid contains the alkali so that the pH value is adjusted to be greater than 10.2 and not greater than 12. Therefore, the polishing rate for the glass substrate can be increased while the high smoothness is maintained. In the existing method, the grinding step is followed by the multi-stage polishing steps using the cerium oxide abrasive grains and the final polishing step in which precision polishing is carried out by the use of the colloidal silica abrasive grains. On the other hand, in this invention, only a single polishing step precedes a final polishing step or no polishing step is required prior to the final polishing step. According to this invention, the productivity is improved and yet the glass substrate having a high smoothness is obtained.

[0022] In case where the abrasive liquid has a pH value not greater than 10.2 and exhibits an alkalinity, the polishing rate for the glass substrate is lowered so that the productivity is decreased. Therefore, the pH value not greater than 10.2 is unfavorable. If the pH value exceeds 12, the colloidal silica abrasive grains are dissolved and precision polishing can not be carried out. Therefore, the pH value exceeding 12 is unfavorable.

[0023] If the pH value of the abrasive liquid becomes small, the colloidal silica abrasive grains may be agglomerated so that the polishing may not effectively be carried out.

[0024] As the alkali for adjusting the pH value of the abrasive liquid, use may be made of, for example, NaOH or KOH.

[0025] If the pH value of the abrasive liquid is adjusted by NaOH, the polishing is effectively carried out. Therefore, use of NaOH is preferable.

[0026] Preferably, the abrasive liquid has a pH value between 10.5 and 11, both inclusive, in view of the productivity and the alkali resistance of a polishing cloth.

[0027] The average grain size of the colloidal silica abrasive grains is appropriately adjusted depending upon the smoothness (surface roughness) of the substrate to be obtained after the polishing. For example, the average grain size is between 0.02 and 0.5 μm. In order to obtain a glass substrate having a high smoothness (surface roughness Rmax≦3 nm) enabling high-density recording/reproducing desired in recent years, the average grain size of the colloidal silica abrasive grains must be equal to or smaller than 0.5 μm. The surface roughness Rmax is defined in Japanese Industrial Standard JIS B0601 as a maximum height representative of a difference between a highest point and a lowest point of a surface.

[0028] The concentration of the colloidal silica abrasive grain is appropriately adjusted depending upon the polishing rate or the surface roughness. For example, the concentration is preferably between 20 wt % and 35 wt %, taking the polishing rate into consideration. As the concentration of the abrasive grains becomes higher, the surface of the substrate is roughened.

[0029] The material of the glass substrate used in this invention is not specifically restricted. For example, use may be made of a quartz glass, a soda lime glass, an aluminosilicate glass, a borosilicate glass, an alumino borosilicate glass, an alkali-free glass, and a crystallized glass.

[0030] According to the above-mentioned structure 2, the polishing by the abrasive liquid containing the colloidal silica abrasive grains and the alkali is followed by the alkali cleaning. In this manner, an abrasive residue of the colloidal silica can be reliably removed. In addition, it is possible to relieve the load (alkali concentration or pH value) upon the glass substrate by the alkali cleaning carried out for the purpose of dissolving and removing the colloidal silica abrasive grains. Therefore, occurrence of the concave defect can be suppressed. Preferably, the alkali concentration in the alkali cleaning after the polishing is greater than the alkali concentration in the polishing step in order to remove the abrasive residue. Specifically, the alkali concentration in the alkali cleaning preferably has a level corresponding to the pH value not smaller than 14. However, the alkali concentration (pH value) must be selected so that no damage (concave defect) is produced in the glass substrate due to the alkali cleaning.

[0031] As a result of the inventors' studies in the light of the above-mentioned viewpoints, it is desired that the alkali cleaning of the glass substrate after the polishing is carried out by the use of the cleaning liquid having a pH value falling within a range between 13.87 and 14.20.

[0032] According to the above-mentioned structure 3, the same alkali component, in particular, NaOH is used as the alkali contained in the abrasive liquid and the alkali contained in the cleaning liquid for the alkali cleaning after the polishing. Use of NaOH is advantageous because an adhered substance, such as the abrasive residue, present on the surface of the substrate can effectively be removed.

[0033] In case where NaOH is used in the alkali cleaning after the polishing, the concentration of NaOH contained in the cleaning liquid preferably falls within a range between 3 wt % and 5 wt %.

[0034] According to the above-mentioned structure 4, the aluminosilicate glass is used as a material of the glass substrate. Use of the aluminosilicate glass is advantageous because the aluminosilicate glass is excellent in chemical durability against an alkali so that the occurrence of the concave defect can be suppressed.

[0035] As the aluminosilicate glass excellent in chemical durability against an alkali, use may be made of a glass having a composition of 58-75 wt % SiO₂, 5-23 wt % Al₂O₃, 3-10 wt % Li₂O, and 4-13 wt % Na₂O.

[0036] According to the above-mentioned structure 5, the magnetic recording medium is produced by forming at least a magnetic layer on the glass substrate obtained according to any one of the structures 1 to 4. Thus, the magnetic recording medium enabling high-density recording/reproducing can be obtained with high productivity. In addition, the magnetic recording medium is prevented from occurrence of an error.

[0037] In this invention, the material of the magnetic layer is not specifically restricted. For example, use may be made of CoCrPtB, CoCrPtTa, CoCrPtNi, CoCrPt, CoCrNiTa, CoCrTa, CoCrNi, and CoCrPtTaB.

[0038] Depending upon the magnetic characteristics, the durability against a magnetic head, and the sliding characteristics, a seed layer, an underlying layer, an intermediate layer, a protection layer, and a lubricating layer may be formed, if appropriate.

[0039] The seed layer serves to control a crystal grain size of a layer formed thereon. For example, the seed layer may be made of a material such as NiAl, AlCo, CrTi, or CrNi.

[0040] The underlying layer generally has a bcc structure and mainly serves to improve magnetostatic characteristics. For example, the underlying layer may be made of a material such as Cr and a Cr alloy (such as CrMo, CrV, CrTi, CrW, and CrTa).

[0041] The intermediate layer generally has a hcp structure and serves to adjust a crystal orientation of the magnetic layer having an hcp structure. For example, the intermediate layer may be made of a material such as CoCr, CoCrB, CoCrPt, and CoCrPtTa.

[0042] The protection layer serves to provide the durability against the magnetic head and the corrosion resistance. For example, the protection layer may be made of a material such as carbon, carbon hydride, carbon nitride, SiO₂, and ZrO₂.

[0043] The lubricating layer serves to improve the characteristics against the magnetic head. For example, the lubricating layer may be made of a perfluoro polyether lubricant.

BRIEF DESCRIPTION OF THE DRAWING

[0044] A sole figure is a graph showing the relationship between a pH value of an abrasive liquid and a polishing rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0045] Hereinafter, this invention will be described in conjunction with several specific examples.

EXAMPLE 1

[0046] This example includes a plurality of steps, i.e., (1) a rough grinding step, (2) a shaping step, (3) an end-face polishing step, (4) a fine grinding step, (5) a first polishing step, (6) a final polishing step, (7) a post-polishing cleaning step, (8) a chemical strengthening step, (9) a post-strengthening cleaning step, and (10) a magnetic disk producing step. Each of the above-mentioned steps will presently be described.

[0047] (1) Rough Grinding Step

[0048] At first, a molten glass was subjected to direct pressing by the use of an upper mold, a lower mold, and a body mold to obtain a disk-shaped glass substrate made of an aluminosilicate glass and having a diameter of 66 mm φ and a thickness of 1.2 mm. Instead of the direct pressing, the disk-shaped glass substrate may be obtained by forming a sheet glass by a down draw method or a float method and then cutting the sheet glass by a grindstone. As the aluminosilicate glass, use was made of a chemically-strengthened substrate glass essentially containing 58-75 wt % SiO₂, 5-23 wt % Al₂O₃, 3-10 wt % Li₂O, and 4-13 wt % Na₂O.

[0049] Subsequently, the glass substrate was subjected to a grinding step. The grinding step is intended to improve the dimensional accuracy and the profile accuracy. The grinding step was carried out by the use of a double-sided grinder with abrasive grains having a grain size of #400. More specifically, the grinding step was carried out by the use of alumina abrasive grains having a grain size of #400 and by rotating inner and outer gears under the load L of about 100 kg. Thus, opposite surfaces of the glass substrate received in a carrier were ground to a surface accuracy between 0 to 1 μm and a surface roughness of about 6 μm in Rmax.

[0050] (2) Shaping Step

[0051] Next, by the use of a cylindrical grindstone, the glass substrate was bored at its center. In addition, an outer peripheral end face of the glass substrate was ground so that the diameter of the glass substrate becomes equal to 65 mm φ. Thereafter, the glass substrate was chamfered at its outer and inner peripheral end faces. At this time, the inner and the outer peripheral end faces of the glass substrate had a surface roughness of 4 μm in Rmax.

[0052] (3) End-face Polishing Step

[0053] Subsequently, the glass substrate was rotated and the inner and the outer peripheral end faces of the glass substrate were polished by brushing to the surface roughness of 1 μm in Rmax and 0.3 μm in Ra. After the polishing, the surfaces of the glass substrate was cleaned with water.

[0054] (4) Fine Grinding Step

[0055] Next, by the use of abrasive grains having a grain size of #1000, the surfaces of the glass substrate were ground to a flatness of 3 μm, the surface roughness of 2 μm in Rmax and 0.2 μm in Ra. Rmax and Ra were measured by an atomic force microscope (AFM) (Nanoscope manufactured by Digital Instruments). The flatness was measured by a flatness meter as a distance (difference in height) between a highest portion and a lowest portion of the surface of the substrate in the vertical direction (which is orthogonal to the surface). After the fine grinding step, the glass substrate was cleaned by successively immersing the glass substrate in cleaning tanks respectively filled with a neutral detergent and water.

[0056] (5) First Polishing Step

[0057] Next, the glass substrate was subjected to a polishing step. The polishing step is intended to remove a flaw and a distortion remaining after the above-mentioned grinding step. The polishing step was carried out by the use of a double-sided polishing apparatus. More specifically, the polishing step was carried out by the use of a hard polisher under the following polishing condition.

[0058] Abrasive Liquid: cerium oxide (average grain size: 1.5 μm)+water

[0059] Load: 80-100 g/cm²

[0060] Polishing Time: 30-50 minutes

[0061] Removed Amount: 35-45 μm

[0062] After the first polishing step, the glass substrate was successively dipped into cleaning tanks respectively filled with a neutral detergent, pure water, pure water, IPA (isopropyl alcohol), and IPA (vapor dry).

[0063] (6) Final Polishing Step

[0064] Next, by the use of a double-sided polishing apparatus of the type same as that used in the first polishing step, the final polishing step was carried out by the use of a soft polisher under the following polishing condition.

[0065] Abrasive Liquid: colloidal silica (average grain size: 0.15 μm,

[0066] concentration: 32 wt %)+NaOH (concentration:

[0067] 1 mol/l, amount: 400 ml)+water

[0068] (pH: 10.5 (measured by pH Scan manufactured by EUTEC))

[0069] Load: 60-120 g/cm²

[0070] Polishing Time: 5-40 minutes

[0071] Removed Amount: 0.5-8 μm

[0072] As shown in the sole figure, the polishing rate was 0.07 μm/min.

[0073] (7) Post-Polishing Cleaning Step

[0074] After the final polishing step, the glass substrate was subjected to alkali cleaning by dipping the glass substrate in an NaOH aqueous solution having a concentration of 3-5 wt %. The cleaning was carried out by applying an ultrasonic wave. Then, the glass substrate was cleaned by successively immersing the glass substrate in cleaning tanks respectively filled with a neutral detergent, pure water, pure water, IPA, and IPA (vapor dry). The surface of the glass substrate thus obtained was observed by an AFM (Nanoscope manufactured by Digital Instruments). As a result, no abrasive residue of colloidal silica was confirmed. Furthermore, an concave defect as a damage due to the alkali cleaning was not observed.

[0075] The NaOH aqueous solution having a concentration of 3-5 wt % had a pH value between 13.875 and 14.097.

[0076] (8) Chemically Strengthening Step

[0077] Next, the glass substrate after the grinding, the polishing, and the post-polishing cleaning steps was subjected to chemical strengthening. For the chemical strengthening, potassium nitrate (60%) and sodium nitrate (40%) were mixed to prepare a chemically strengthening salt. The chemically strengthening salt was heated to 375° C. The glass substrate after cleaned was preheated to 300° C. and dipped in the chemically strengthening molten salt for about 3 hours.

[0078] By dipping the glass substrate in the chemically strengthening molten salt, lithium ions and sodium ions in a surface layer of the glass substrate were replaced by sodium ions and potassium ions in the chemically strengthening salt, respectively. As a result, the glass substrate was strengthened. A compressive stress layer formed on the surface layer of the glass substrate had a thickness between about 100 and 200 m. After chemical strengthened, the glass substrate was rapidly cooled by dipping the glass substrate in a water tank kept at 20° C., and held for about 10 minutes.

[0079] (9) Post-Strengthening Cleaning Step

[0080] After rapidly cooled, the glass substrate was cleaned by dipping the glass substrate in a sulfuric acid heated to about 40° C. and applying an ultrasonic wave. The surface roughness of the glass substrate after cleaned was measured by the AFM (Nanoscope manufactured by Digital Instruments). Then, Rmax and Ra were equal to 2.62 nm and 0.31 nm, respectively. Thus, the result was excellent.

[0081] As a consequence, Rmax/Ra was equal to 8.45. Herein, Rmax and Ra were defined by the Japanese Industrial Standard (JIS) B0601.

[0082] (10) Magnetic Disk Producing Step

[0083] On the glass substrate obtained through the above-mentioned steps, an NiAl seed layer, a CrV underlying layer, a CoCtPtB magnetic layer, and a carbon hydride protection layer were deposited by the use of a sputtering apparatus. Thereafter, by a dipping method, a perfluoro polyether lubricating layer was formed. Thus, a magnetic disk was produced.

[0084] The magnetic disk thus obtained was subjected to a glide height load/unload test (400,000 times). As a result, no head-medium contact occurred to a flying height of 4.5 nm. In addition, no crush occurred. In a recording/reproducing test, no signal error was confirmed.

[0085] In view of the glide height, it is preferred that no head-medium contact occurs to a flying height of 5.0 nm. In other words, it is preferred that the glide height is not greater than 5.0 nm.

EXAMPLE 2

[0086] Next, a glass substrate and a magnetic disk were produced in the manner similar to Example 1 except that the amount of the alkali contained in the abrasive liquid was adjusted to adjust the pH concentration of the abrasive liquid.

[0087] As a result, as shown in the sole figure, the polishing rate for the glass substrate was improved as the pH concentration was higher. In case where the pH concentration of the abrasive liquid exceeds 12, colloidal silica was dissolved and polishing could not be carried out.

[0088] The glass substrates were obtained at different pH concentrations of the abrasive liquid within a range not higher than 12. For these glass substrates, the surface roughness was measured by the use of the AFM (Nanoscope manufactured by Digital Instruments). As a result, it was confirmed that the surface roughness was equivalent to that achieved in Example 1 and was not substantially changed.

[0089] Like Example 1, no abrasive residue of colloidal silica on the surface of the glass substrate was confirmed. In addition, an concave defect as a damage due to the alkali cleaning was not observed.

[0090] The magnetic disks thus obtained were subjected to a glide height load/unload test (400,000 times). As a result, no head-medium contact occurred to a flying height of 4.5 nm. In addition, no crush occurred. In a recording/reproducing test, no signal error was confirmed.

Comparative Example 1

[0091] Next, a glass substrate and a magnetic disk were produced in the manner similar to Example 1 except that NaOH was not added to an abrasive liquid (having a pH concentration of 10.2) in the final polishing step. In order to avoid presence of the abrasive residue of colloidal silica, the NaOH concentration in the alkali cleaning was 7 wt % and the ultrasonic wave was applied strongly.

[0092] When the NaOH concentration in the alkali cleaning was 7 wt %, the pH value was equal to 14.243.

[0093] The result of Comparative Example 1 is shown in the sole figure.

[0094] As illustrated in the sole figure, the polishing rate was lowered to 0.04 μm/min. Accordingly, in order to achieve the surface roughness equivalent to Examples, a longer polishing time is required. The increase in polishing time corresponds to 100 glass substrates as a decrease in number of glass substrates produced per one hour. If three-stage polishing steps are performed as disclosed in JP 10-241144 A, the increase in polishing time corresponds to 150 glass substrates as a decrease in number of glass substrates produced per one hour.

[0095] In Example 1, the polishing rate was 0.07 μm/min. For example, if the polishing amount required in polishing is equal to 1.5 μm, the polishing time of 21.4 minutes is required.

[0096] On the other hand, in Comparative Example 1, the polishing rate was 0.04 μm. For example, in order to remove the polishing amount of 1.5 μm, the polishing time of 37.5 minutes is required.

[0097] In case where a polishing apparatus for simultaneously polishing 100 glass substrates is used, about 300 glass substrates can be polished in one hour in Example 1. On the other hand, in Comparative Example 1, no more than about 100 to less than 200 glass substrates can be polished in one hour so that the production cost is increased.

[0098] In this invention, the polishing rate not lower than 0.05 μm/min is preferable. In this case, in order to remove the polishing amount of 1.5 μm, the polishing time of 30 minutes is required. Therefore, if the polishing apparatus for simultaneously polishing 100 glass substrates is used, it is possible to polish 200 glass substrates per one hour.

[0099] In Comparative Example 1, no abrasive residue of colloidal silica on the surface of the glass substrate was observed. However, the alkali (NaOH) concentration was high and the cleaning condition (application of ultrasonic wave) was strong in order to remove the abrasive. As a result, an concave defect as a damage due to the alkali cleaning was observed.

[0100] The magnetic disk was subjected to a glide height load/unload test. As a result, no substantial difference was observed as compared with Examples. However, in a recording/reproducing test, a signal error due to the concave defect was confirmed.

EXAMPLE 3

[0101] A glass substrate was produced in the manner similar to Example 1 except that NaOH was replaced by KOH as the alkali contained in the abrasive liquid in the final polishing step (6). The concentration of KOH was adjusted so that the abrasive liquid has a pH value of 10.8.

[0102] As a result, the polishing rate was 0.07 g m/min.

[0103] The surface roughness of the glass substrate was measured in the manner similar to Example 1. Then, Rmax and Ra were equal to 2.91 nm and 0.29 nm, respectively. Therefore, Rmax/Ra was equal to 10.

[0104] No abrasive residue and no concave defect as a damage due to the alkali cleaning were confirmed.

[0105] The glass substrate was subjected to the magnetic disk producing step (10) in the manner similar to Example 1. A magnetic disk thus obtained was tested in the manner similar to Example 1. As a result, no head-medium contact occurred to a flying height of 4.8 nm. In addition, no crush occurred. In a recording/reproducing test, no signal error was confirmed.

[0106] Comparing the results in Examples 1 and 3, the result of Example 3 was also good. However, the glide height was 4.5 nm in Example 1 while the glide height was slightly degraded to be 4.8 nm. This is presumably because Rmax/Ra is not smaller than 10.

[0107] In view of the glide height, Rmax/Ra is preferably smaller than 10 in this invention.

[0108] The reason why the glide height was slightly degraded in Example 3 as compared with Example 1 was investigated. In visual observation, no difference was confirmed. However, as a result of precision inspection using a scanning electron microscope (SEM), it was confirmed that a very small amount of the abrasive residue was left on the surface of the substrate.

Reference Example

[0109] Next, a glass substrate and a magnetic disk were produced in the manner similar to Example 1 except that the glass substrate was made of a quartz glass and that the concentration of NaOH contained in the abrasive liquid was varied.

[0110] As a result, the polishing rate was not substantially changed even if the pH concentration was varied. No concave defect as a damage due to the alkali cleaning was caused.

[0111] The magnetic disk was subjected to a glide height load/unload test and a recording/reproducing test. The results were similar to those in Examples.

[0112] Comparing the results in Examples and Reference Example, it has been confirmed that the aluminosilicate glass is suitable because the polishing rate can be adjusted considering the productivity and the chemical durability against the alkali is excellent.

[0113] According to this invention, it is possible to obtain a glass substrate for a magnetic recording medium, which has a high smoothness without an abrasive residue and an concave defect and to achieve a high recording density of a magnetic recording medium. 

What is claimed is:
 1. A method of producing a glass substrate for a magnetic recording medium, comprising the steps of grinding a principal surface of said glass substrate of a disk-shape and polishing, after said grinding step, the principal surface of said glass substrate by an abrasive liquid including abrasive grains, wherein said abrasive grains are colloidal silica abrasive grains, said abrasive liquid comprising an alkali so that a pH value of said abrasive liquid is adjusted to be greater than 10.2 and not greater than
 12. 2. A method as claimed in claim 1, further comprising the step of carrying out, after said polishing step, alkali cleaning of the principal surface of said glass substrate.
 3. A method as claimed in claim 2, wherein the same alkali component is used as the alkali included in said abrasive liquid and an alkali used in said alkali cleaning.
 4. A method as claimed in claim 1, wherein said glass substrate is made of an aluminosilicate glass.
 5. A method of producing a magnetic recording medium, comprising a step of forming at least a magnetic layer on the principal surface of said glass substrate which is obtained by the method as claimed in claim
 1. 